Plant cultivation using artificial light sources is generally performed in a flow in which seeds are first set for germination (seeding period), seedlings are then grown (seedling growth period), grown seedlings are planted in a medium (planting period), and plants grown to an appropriate size are harvested (harvest period).
Photosynthesis of plants progresses in accordance with a cycle of a light reaction that requires light energy and a dark reaction that does not require light energy, as disclosed in Patent Literature 1. According to past researches, it is said that the cycle of the light reaction and dark reaction is about 200 to 400 μsec period (see Non-Patent Literature 1). It may therefore be possible to significantly save the power consumption by controlling exposure of plants to light to match the above-described period so that the plants are exposed to light only during the light reaction and are not exposed to light during the dark reaction. For example, setting the duty ratio of exposure to light at 50% allows the power consumption to be half as compared with the case of continuous exposure.